Accessing /forums/ returns /forums/index.php no problem.
I thought the problem might have been caused by the fact that the /music/ URI was used as a stub for a WordPress-created page in the past, even though I'd since deleted it. So I renamed the now real directory to /musiczzz/: no change.
I followed someone's advice to stick the all-encompassing WordPress directives at the end of .htaccess, not the start: no change.
I can't wrap my head around the .htaccess directives, despite the fact they don't deviate from the supposedly docile default, but still they seem to be hijacking Apache to distraction.
I'll upgrade to version 2.1 of WordPress, but I don't think that'll make any difference either.